1. Field of the Invention
The present invention relates to a zoom lens barrel, and more specifically, is directed to a rear focusing zoom lens barrel for use in 35 mm single-lens reflex cameras.
2. Related Background Art
As a focusing system for a zoom lens, known is a front group shift focusing, a rear group shift focusing, and other focusing systems.
In the zoom lens having a wider range of object angle, a first lens group which is located at the position nearest to the object has generally a larger lens diameter. Consequently, the front group shift focusing system in which the first lens group is shifted is liable to suffer a substantially heavy load. Accordingly at an electric automatic so-called AF focusing, the focusing speed is decelerated.
On the contrary, the rear group shift focusing system in which a rear group lens facing the mount side is shifted toward the film surface, that is, so-called rear focusing system is less loaded when shifting the lens groups. As a result, at AF focusing, a relatively prompt focusing speed can be obtained as compared to the front group shift focusing system.
A great variety of techniques with regard to the lens configuration in which the rear focusing system is employed for the zoom lens are known. Above all, Japanese Patent application Laid-Open No. 60-43619 discloses a structure in which a shift of lens group during the focusing action and a shift during the zooming action are performed by the same member.
In the conventional rear focus zoom lens, when the focusing operation is executed from the farthest side to the nearest side, the rear lens group is backwardly moved toward the camera body side. At this time, if the shift distance is too large, the lens collides with a jump-up mirror provided inside the camera body. Therefore, a shift limit is provided so as to avoid a collision to prevent a further retreat of the rear lens group. This often prevents the rear lens group from shifting up to the closest distance at which the performance on the optical design is ensured. When the shift limit for avoiding a collision arising from retreat of the rear group lens is uniformly located over the entire focal length range of the zooming field, the shift limit must be positioned corresponding to a narrower focal length range of the zooming field. In consequence, even in the other field in which there is no risk that the rear lens group collides with the jump-up mirror, since they are remote enough from each other as compared with the case with the aforementioned focal length range, the closest photographing length is needlessly restricted in the same manner.
Due to this, across the overall focal length range of the zooming field, the performance on the optical design can not satisfactorily be exhibited.